Radiation is energy that propagates through matter or space. Radiation energy can be electromagnetic or particulate. Radiation is usually classified into non-ionizing (visible light, TV, radio wave) and ionizing radiation. Ionizing radiation has the ability to knock electrons off of atoms, changing its chemical properties. This process is referred to ionization (hence the name, ionizing radiation). Ionizing radiation is the main concern for health effects since it can change chemicals’ properties in the human body.

Radiation comes from many sources including cosmic rays from the universe, the earth, as well as man-made sources such as those from nuclear fuel and medical procedures. Radiation has been used in many industries including diagnostic imaging, cancer treatment (such as radiation therapy), nuclear reactors with neutron fission, radioactive dating of objects (carbon dating), as well as material analysis.

Ionizing radiation and its effects on the human body

There are four main types of ionizing radiation: electrons (also known as beta), photons (mostly gamma ray and X-ray), charged particles (alpha) and neutrons. In a nuclear reactor, the radiation is formed due to the decay of radioactive isotopes, which are produced as part of nuclear reactions inside the reactor.

Each ionizing radiation type interacts with the body differently but the end results are similar. When radiation enters a body, it can deposit enough energy that can directly damage DNA or cause many ionizations of atoms in tissues that would eventually cause damage to critical chemical bonds in the body. The mechanisms of how radiation damages tissues and the degree of damage of each type of radiation are different. However, the amount of radiation needed to cause permanent damage to the tissue depends on the total dose to the body, the type of radiation, and the amount of time it takes to get that amount of radiation (dose rate). Also, depending on the total dose and/or dose rate, the effect can be acute (happen right away such as radiation burns, sickness, nausea) or delayed (long-term, such as cancer ).

What are the health effects of various doses/dose rates?

Radiation dose is measured in Rad or Gy (1Gy = 100 Rad). However, the most often reported two units that have been mentioned in the media are Sievert (Sv) and Rem (1 Sv = 100 Rem). These are defined as dose equivalent, which accounts for the different effects each type of radiation have on the body. The Sievert and Rem are units used by regulatory authorities to control radiation release and exposure. The table below lists the different amount of radiation you can get from your normal activities.

It is important to note that the health effects of radiation exposure vary for different doses. It is important to note dose is different from dose rate. Dose refers to the total amount of exposure, while dose rate refers to the exposure per unit of time (typically per hour). The dose numbers provided in the following discussion are not exact numbers, but instead general averages. An acute dose (received in a few days) above 250-400 Rem (2.5 – 4.0 Sv) is considered to be lethal for at least half of the population exposed. Not much is known about doses between 50 Rem and 250 Rem (500 mSv and 2500 mSv), but the exposed person will experience acute radiation sickness. The symptoms of such exposure can include nausea, vomiting, diarrhea, burns, and hair loss, but may or may not lead to near term death. Below this level, no acute symptoms have been observed. For radiation exposure of less than 50 Rem there is the potential for delayed effects such as non-specific life shortening, genetic effects, fetal effects, and cancer, but little is known about the long term consequences of exposures in this range. For doses less than 25 Rem there are not enough data to determine if such an exposure can cause any long-term effects on human health at all.
Lethal radiation dose compared to dose from normal activities. Again, these numbers reflect cumulative dose, not dose rates. To determine cumulative dose, multiply the dose rate by the time exposed:

Cumulative Dose = Dose Rate x Time Exposed

Radiation released from reactors at Fukushima and what it means

The radioactive fission products from the affected reactors include noble gases (xenon and krypton), volatile radioactive isotopes (iodine-131 and cesium-137) and non-volatile fission products. As mentioned before, these radioactive products release radiation as they decay. Therefore, over exposure and/or contact with them is dangerous. The noble gases are usually not of a big concern since they are inert, and tend to impose very small doses. Non-volatile fission products usually stay within the fuels so that is not much of a concern to the general public either. The fission products of most concern are the volatile ones such as I-131 and Cs-137 since they can be dispersed in air and get carried far away by wind from the affected reactors.

Iodine-131 is a radioactive isotope that releases beta particles (electrons). Concentration of iodine-131 in the thyroid has been shown to cause thyroid cancer. Therefore, it is a big concern if too much iodine-131 gets out of the reactor and falls to the ground away from the affected reactors. This can contaminate food, water, and animal products such as milk. The Japanese government has distributed iodine pills to people in the affected area. These iodine pills contain stable iodine-127, which does not cause cancer. When people take these iodine pills their bodies absorb the stable iodine to a level that prevents or limits the absorption of I-131, which helps to prevent the risk of thyroid cancer. Another fact about radioactive iodine-131 is that its half-life (the time it takes for half of it to decay to another nuclear isotope) is only about 8 days. This means that after about three months, almost all of the radioactive iodine-131 would have decayed away.

Cs-137, also emits a beta particle as it decays. Exposure to Cs-137 can also increase the risk of getting cancer but that again depends on the dose and the dose rate. However, Cs-137 causes a much longer term contamination problem because its half-life is about 30 years. Depending on the amount of Cs-137 that is released, and the regulations for acceptable elevated background radiation levels, the area contaminated with Cs-137 may not be inhabitable for a long time.

How to minimize radiation exposure

The rules of thumb for minimizing your exposure are to use time, distance, and shielding to your advantage. Shorten the time of your exposure to radiation, stay as far away from the radioactive source as reasonably possible (radiation goes down quickly as a function of distance, ~1/r2), and provide more shielding between you and the source. This is one of the reasons the people very close to the reactors were required to evacuate very early on after the earthquake. Also, the government recommended people between 20 and 30 km to stay indoors (because their houses provide extra shielding from some of the radiation – beta, alpha), and minimize their time outdoors to limit their exposure.

We strongly urge that our readers in the region follow the instructions of their local governments regarding if, when, and how to take cover or evacuate.